In general, servo control apparatuses (such as velocity control apparatuses and position control apparatuses) used for controlling axes in numerically controlled machines are required to have good system stability (including a vibration suppressing capability), and high levels of command following performance and disturbance suppressing performance. FIG. 6 shows a general configuration of a servo system in one of such servo control apparatuses. In FIG. 6, a servo control apparatus constitutes a feedback system in which a controller C is used and a disturbance d is applied to an input u of a target plant 55, to thereby cause an output y of the target plant 55 to follow a command value Y. In this servo system, the command following performance is improved as a bandwidth (a range in which |T|≈1) of a complementary sensitivity function T=C˜P/(1+C˜P) representing a transfer characteristic of Y→y becomes broader, while the disturbance suppressing performance to suppress the disturbance d is improved as a value of |S| in a middle to low frequency range of a sensitivity function S=1/(1+C˜P) representing a transfer characteristic of d→u becomes smaller.
Here, the command following performance may be improved by forming a feedforward system (not illustrated). Meanwhile, when the sensitivity function S in the middle to low frequency range is minimized to improve the disturbance suppressing performance, the bandwidth of the complementary sensitivity function T is inevitably broadened to a high range. This often results in problems such as occurrence of vibrations in a high frequency band and a decrease in system stability. To circumvent such problems, a control method for suppressing only disturbance in a minor loop has been conventionally suggested. As a conventional disturbance suppressing control method, there has been known a control method (hereinafter referred to as a disturbance observer method) of approximating a target plant to a lower order model, estimating a disturbance d input to the target plant, and providing to a control input feedback data for cancelling the disturbance d, to thereby suppress disturbance.
FIG. 7 is a block diagram showing an example of a conventional position control apparatus to which the disturbance observer method is applied. The target plant 55 is actuated by a drive motor (not illustrated), and a control input u of the target plant 55 receives a signal obtained by adding the disturbance d to a motor generated drive force uc in an adder 54. An output v from the target plant 55 represents a motor velocity detected by a detector (not illustrated) or the like, and another output x from the target plant 55 is composed of a driver position and a motor position detected in a manner similar to that used by the detector. A velocity control unit 100 is a velocity control apparatus for controlling a motor velocity v exactly in accordance with a velocity command value Vc, and is configured to amplify, in a velocity deviation amplifier Gv, a velocity deviation signal obtained by subtracting the motor velocity v from the velocity command value Vc.
A disturbance observer 53 internally stores a lower order model of the target plant 55 and outputs a disturbance estimate value ^d calculated from the motor generated drive force uc and the motor velocity v. A drive force command value u0, which is an output from the velocity deviation amplifier Gv, is passed as the motor generated drive force uc obtained by subtracting the disturbance estimate value ^d from the drive force command value u0 in a subtractor 52. To control a driver position x (or a motor position indirectly indicative of the driver position) exactly in accordance with a position command value Xc, a position control apparatus 101 subtracts the driver position x from the position command value Xc in a subtractor 50 to find a position deviation and amplifies the position deviation in a position deviation amplifier Gp. An amplified output from the position control apparatus 101 is passed as the velocity command value Vc to the velocity control unit 100.
In the conventional position control apparatus to which the disturbance observer method as described above is applied, so long as a transfer characteristic ˜P of the target plant matches a transfer characteristic P of the lower order model stored in the disturbance observer, the disturbance estimate value ^d is obtained as a precise estimate value of the disturbance d, and the disturbance d can be cancelled by feeding back the precise disturbance estimate value ^d. In this way, disturbance suppressing performance can be improved without exerting any influence on the command following performance. However, because the transfer characteristic ˜P of the target plant generally does not match the transfer characteristic P of the lower order model, especially in a high frequency band, the disturbance estimate value ^d to be fed back may include data of a sort of unintended state feedback. This often exerts an adverse influence on the command following performance, and results in occurrence of vibrations.
Referring to FIG. 8, control characteristics associated with the thus-configured conventional position control apparatus are described more specifically. FIG. 8 is a block diagram showing, in detail, an exemplary configuration of a velocity control unit 100. Description of the components explained with reference to FIG. 7 is not repeated below. Here, the transfer characteristic ˜P of the target plant is assumed to be a transfer characteristic from the control input u to the motor velocity v, and defined as a two-inertial system having a transfer pole cop and a transfer zero ωz expressed by equation (1) as follows.˜P=(s2+ωz2)/{I1s(s2+2ζωp·s+ωp2)}  (1)
Where I1 represents a drive side moment of inertia, and ζ represents an attenuation factor. Then, the lower order model stored in the disturbance observer is defined as a one-inertial system, and the transfer characteristic P of the model is defined so as to match the transfer characteristic ˜P of the target plant in a low frequency range by equation (2) as follows.P=1/(Is)=1/{(I1+I2)s}  (2)
Where I2 represents a load side moment of inertia, and an equivalent moment of inertia I is equal to (I1+I2).
Operation of outputting the disturbance estimate value ^d performed by the disturbance observer 53 may be equivalently expressed by an operation of subtracting, in a subtractor 56, the motor generated drive force uc from the motor velocity v multiplied by an inverse transfer characteristic P−1 of the lower-order model, and multiplying a subtracted result by a transfer characteristic K0 in a transfer function block 57. The transfer characteristic K0 in the transfer function block 57, which may be expressed using an observer gain ω0 as K0=I·ω0, serves as a primary low pass filter characteristic in below-described equation (3).PK0/(1+PK0)=(ω0/s)/{1+(ω0/s)}=ω0/(s+ω0)  (3)
Here, parameters associated with the transfer characteristic ˜P are set as I1=0.2 [kgm2], I2=0.4 [kgm2], ζ=0.005, ωp=628 [rad/s], and ωz=364 [rad/s], and the velocity deviation amplifier Gv is suitably determined on condition that the disturbance observer method is not applied (ω0=0). Frequency characteristics (Vc, d→v, v2) of the velocity control unit 100 obtained using the above parameters and the velocity deviation amplifier Gv are shown in FIG. 9. It should be noted that v2 represents the load side velocity of the target plant 55 expressed with the two-inertial system, and the velocity deviation amplifier Gv is configured by a generally-used proportional integral compensator. In comparison, FIG. 10 shows the frequency characteristics (Vc, d→v, v2) obtained when the disturbance observer method is employed with the observer gain ω0 selected as ω0=300 [rad/s].
The disturbance suppressing performance represented by the frequency characteristics (d→v, v2) is improved in the middle to low range by applying the disturbance observer method. However, as is evident from FIG. 10, in a frequency band close to the transfer zero ωz where a difference between the transfer characteristic ˜P of the target plant and the transfer characteristic P of the lower order model is maximized, application of the disturbance observer method affects the command following performance represented by the frequency characteristic (Vc→v2) with respect for which a gain is increased, resulting in occurrence of vibrations.
How the command following performance is affected by the disturbance observer method may be understood as described below. Because the transfer characteristic K0 in the transfer function block 57 of FIG. 8 has a dropping characteristic of only −20 dB/dec at maximum as shown in equation (3), the transfer function block 57 is unable to cut off increasing components other than the disturbance contained in an input to the transfer function block 57 in the frequency range where a plant error, which is the difference between the transfer characteristics ˜P and P, is increased.
To grasp operation of a disturbance suppressing system in FIG. 8, a disturbance suppressing part is equivalently converted assuming that u0=0. FIG. 11 shows a result of this equivalent conversion. When the transfer characteristic ˜P of the target plant is expressed using a nominal characteristic of the transfer characteristic P and the plant error, the problem of reducing the magnitude of equation (3) with respect to the plant error may be replaced by a robust stability problem. This is used in JP H11-24708 A for designing the transfer characteristic K0 with H∞ control design (where K0 is an H infinity controller).
In JP H11-24708 A, however, a velocity control unit is structured as shown in FIG. 12, which causes a response to the drive force command value u0 to become equivalent to a response to the disturbance d in the disturbance suppressing system. Accordingly, when this disturbance suppressing system is introduced, a command following characteristic undergoes a change in the middle to low range. To prevent the change, the command following performance should not be designed independently of the disturbance suppressing performance, which inevitably involves redesign of the velocity deviation amplifier Gv.
As described above, with the conventional servo control apparatus including the disturbance suppressing system which is formed in the minor loop with the intention of improving disturbance suppressing performance, the disturbance suppressing performance may be improved in the middle to low range, while the disturbance suppressing system affects the command following system and therefore induces vibrations or other adverse influences. Due to these influences, it has been impossible to separately design the command following performance and the disturbance suppressing performance. There is a need for a servo control apparatus including a disturbance suppressing system capable of solely enhancing disturbance suppressing performance in the middle to low range without having an influence on the command following system.